The sensing and measuring of liquid levels is essential in many applications. For example, and without limitation, many water reservoirs need water level to be monitored so that they can be refilled, or to prevent overflow and flooding. Also, environmental sensing of water level in rivers, oceans, and lakes is becoming increasingly more important with climate change, and as demand for water resources increases.
Many methods and devices for measuring liquid levels have been used. The most common is the mechanical float, which consists of buoyant floats coupled with water valves, electronic switches, rotary sensors, or magnets and magnetic sensors that capture position level as the float moves up and down. The problem with these types of measuring devices is that they are mechanical and have moving parts which may break or deteriorate over time and with use, are bulky and such devices which measure a continuum of levels are often very expensive.
Resistive sensors (see, U.S. Pat. No. 4,137,527; U.S. Pat. No. 4,276,454; U.S. Pat. No. 7,992,437; U.S. Pat. No. 5,719,556; U.S. Pat. No. 3,916,213), which consist of conductive probes inserted into a conductive liquid are often used, but they suffer from corrosion and can cause electrolysis when used in water. They are also sensitive to salinity and can ignite combustible liquids.
Another common technology is capacitive sensors. These sensors primarily consist of one or more plates arranged in various geometries and shapes. The plates can be oriented inside or outside a reservoir. They can consist of parallel plates or concentric tubes (see, U.S. Pat. No. 4,924,702). Some use multiple staggered plates (see US 2009/0148306 A1). Liquids tend to have a higher dielectric constant than air or gasses. Water in particular has a very high dielectric constant of 80, and is easily measured with dielectric techniques. When liquid level changes across a capacitive plate, the capacitance changes proportionally. Each of these capacitive sensors employ many different methods of electronically determining the capacitance changes of the plates, and thus the liquid level. The disadvantage of this type of sensor is that large plates are difficult and expensive to produce so measuring large ranges of water level becomes prohibitively expensive.
Yet another less common but sophisticated technique, known as TDR, uses transmission lines to measure the time delay of pulse reflections from the liquid to air boundary (see, U.S. Pat. No. 3,995,212; U.S. Pat. No. 5,610,611). While the length of the sensor is unbounded, the electronics employed to measure the time delay of reflected pulses is often prohibitively expensive.
In view of the foregoing, there is a need to provide a level sensing apparatus and method, which is inexpensive to manufacture, easy to use, measures a wide range of liquid levels, and is easy to calibrate.